This invention relates to processes and elements for making photoresists with photosensitive thermoplastic materials.
In processes of the prior art, the resist material is applied to the metal or other permanent surface as a liquid coating and then exposed to light either in a liquid gellable state or in a dry state. The coating is done by dipping, whirling or roller coating. This is inconvenient in many cases because of the requirement that either the support, e.g., metal, must be coated at the point of manufacture of the resist composition, or the said composition must be shipped to the user of the resists who must then coat the composition. These piece by piece operations are generally wasteful of resist composition and produce coatings of marginal quality and uniformity. Either procedure has many obvious disadvantages. One of these is that liquid coatings applied to perforated circuit boards in certain cases where electroplating is to be done clog the perforations and prevent making the electrical connections to both sides of the board through the perforations. Another disadvantage is that the preparation of photoresists by the above processes involves long drying times by the user and a high risk of dust particles settling on the coated surface during the drying period.
It has been found that these disadvantages of the prior art may be eliminated by the use of preformed resist films of photosensitive material. More specifically, it has been discovered that photoresists may be produced by laminating a thin layer of a positive-working material to a support and then imaging and developing it. U.S. Pat. No. 3,469,982, issued Sept. 30, 1969, to Celeste, describes a method of making a photoresist by laminating a negative-working, photopolymerizable layer to a support and further treating it so as to produce a resist image. The lamination is carried out by coating the photopolymerizable material on a film support, the upper surface being uncovered or having a protective cover sheet which is removed prior to lamination. The photopolymerizable material is coated on the film support as a very thin layer (e.g., 0.00035 inch), and must accordingly have physical characteristics suitable for transfer as a thin, dry layer, particularly where it is to be rolled up for storage prior to lamination.
Photopolymerizable (i.e., negative-working) photoresist materials are suitable for use in such a process since they contain a substantial proportion of an ethylenically unsaturated photopolymerizable monomer, which presents no problem when transferred as a dry layer. Moreover, the monomer often acts as a plasticizer for the binder that may be used in the photopolymerizable layer (see U.S. Pat. No. 3,469,982, Column 1, lines 35-37). In fact, this plasticization of the binder can occur to such an extent with some monomers as to present the problem of excessive softness (see U.S. Pat. No. 3,261,686 to Celeste and Bauer, issued July 19, 1966, Column 1, lines 41-51, and U.S. Pat. No. 3,380,831 to Cohen and Shoenthaler, issued Apr. 30, 1968, Column 1, lines 64-70).
Just the opposite problem would be expected when using positive-working materials (i.e., not photopolymerizable) in a process similar to that described in U.S. Pat. No. 3,469,982. Positive-working photosensitive materials are solubilized or decomposed, rather than polymerized, in areas exposed to light. Thus, they contain material which is already polymerized and do not contain photopolymerizable monomers. Frequently these materials contain thermosetting resins or highly crosslinked colloids. A dry layer, containing polymer unplasticized by monomer, must therefore be transferred in the process of the invention, rather than a plasticized, monomer-containing layer as in the above-cited patent. The layer must also have adequate flexibility and adhesion to the substrate to which it is laminated. Surprisingly, both transferability and adhesion are achieved with the positive-working photosensitive materials of the invention, even though they contain no photopolymerizable monomer. This is particularly unexpected since positive-working materials are generally ionic or strongly polar and are often water-soluble. Such materials would not be expected to form flexible thermoplastic layers having good adhesion to metal substrates. It has been found, however, that the positive-working materials of this invention are operable in the process of the invention even though they contain no photopolymerizable monomer to plasticize them.